Only appears to happen when using IE9 - Chrome, IE8, Firefox and Opera are all fine and don't get the error

Fiddler trace when testing with IE appears to show no attempts to make requests to or from Paypal, but they are shown in all other browsers.

I am still looking for a fix for this, I will have a look at the redirectWith that Craftnet mentioned, but I'm not really sure what it should be changed to/from. If anyone can shed some light on this issue would be great.